Cross-linking of modified engineering thermoplastics

ABSTRACT

The present invention pertains to a process for the cross-linking of modified engineering thermoplastics, in particular, of polymeric sulfinic acids or sulfinic acid salts.  
     In particular, the invention pertains to a process for the preparation of cross-linked polymers, characterized in that solutions of polymeric sulfinic acids or sulfinic acid salts (—SO 2 Me), optionally in the presence of organic di- or oligohalogeno compounds [R(Hal) x ], are freed from solvent and cross-linked to polymers,  
     wherein  
     Me stands for a monovalent or polyvalent metal cation;  
     R stands for an optionally substituted alkyl or aryl residue containing from 1 to 20 carbon atoms; and  
     Hal stands for F, Cl, Br or I.

[0001] The present invention pertains to a process for the cross-linking of modified engineering thermoplastics, in particular, of polymeric sulfinic acids or sulfinic acid salts.

[0002] Various ways for preparing cross-linked polymers and membranes are known in the prior art. Some of these may be mentioned here:

[0003] 1) Preparation of cross-linked membranes by the copolymerization of mono- and bifunctional monomers (example: copolymerization of styrene and divinylbenzene in thin layers, followed by sulfonation of the cross-linked polymer membrane produced). This method has the disadvantage that the cross-linked ionomer membranes produced thereby have limited oxidation stabilities since both styrene and divinylbenzene contain tertiary C—H bonds which are sensitive to oxidation. (For a comparison of the oxidation sensitivities of styrene having a tertiary C—H bond and of α-methylstyrene without a tertiary C—H bond, see, for example: Assink, R. A.; Arnold C.; Hollandsworth, R. P., J. Memb. Sci. 56, 143-151 (1993). )

[0004] 2) Functionalization of ion-exchanging groups of ionomers, followed by reaction with bi- or oligofunctional cross-linking agents to yield a cross-linked membrane (example: functionalization of sulfonic acid groups to yield the reactive acid chloride or imidazolide, followed by reaction with (aromatic) amines; Nolte, R.; Ledjeff, K.; Bauer, M.; Müllhaupt, R.; J. Memb. Sci. 83, 211 (1993)). In these cross-linking methods, cross-links (sulfonamide groups) are formed in the membrane which are not sufficiently stable to hydrolysis.

[0005] 3) Chemical activation of ion-exchanging groups of ionomers, followed by reaction of the activated group with other groups of the polymer main chain to yield a cross-linked membrane (example: conversion of part of the sulfonic acid groups of sulfolated PEEK (polyetheretherketone) to sulfochloride groups, attack of the sulfochloride groups during membrane formation in the hydroquinone region of the PEEK repeating unit under Eriedel-Crafts acylation and formation of hydrolysis-stable —SO₂— links (EP 0 574 791 A2). This method has the disadvantage that it can be employed only with certain aryl polymers, such as PEEK.

[0006] From Kice, J. L.; Pawlowski, N. E: J. Org. Chem 28, 1162 (1963), it has been known that low-molecular sulfinic acids can disproportionate according to the following scheme of reactions;

[0007] From Quaedvlieg, M., in: Houben-Weyl, Methoden der Organischen Chemie, Vol. II, 606, Thieme Verlag, Stuttgart (1957); Ashworth, M. R. F., in: The Chemistry of Sulphinic Acids, Esters and their Derivatives (Ed.: S. Patai), chapter 4, 97-98, John Wiley Ltd., New York (1990); Allen, P.: J. Org. Chem. 7, 23 (1942), it has been known that sulfinate can readily be alkylated to the sulfonate. The reaction was performed, inter alia, in alcohols having different chain lengths:

[0008] It has, been the object of the present invention to provide novel processes for cross-linking modified engineering thermoplastics, in particular, of polymeric sulfinic acids or salts thereof.

[0009] The above object is achieved, in a first embodiment, by a process for the preparation of cross-linked polymers, characterized in that solutions of polymeric sulfinic acids or sulfinic acid salts (—SO₂Me), optionally in the presence of organic di- or oligohalogeno compounds [R(Hal)_(x)], are liberated from solvent and cross-linked to polymers,

[0010] wherein Me stands for a monovalent or polyvalent metal cation;

[0011] R stands for an optionally substituted alkyl or aryl residue containing from 1 to 20 carbon atoms; and

[0012] Hal stands for F, Cl, Br or I.

[0013] It is particularly preferred that the sulfinic acids or salts thereof is derived from structures comprising aromatic nuclei having R₁ or R₂ structures of the following formulae as the repeating unit wherein

[0014] R₃ stands for hydrogen, trifluoromethyl or C_(n)H_(2n+1), with n being from 1 to 10 , in particular methyl;

[0015] R₄ stands for hydrogen, C_(n)H_(2n+1), with n being from 1 to 10, in particular methyl or phenyl; and

[0016] x stands for 1, 2 and 3,

[0017] which are linked through bridging groups R₅ or R₆ wherein

[0018] R₅ stands for —O—, and

[0019] R₆ stands for —SO₂—.

[0020] It is particularly preferred according to the present invention that the solvent is selected from dipolar-aprotic solvents, such as NMP, DMAc, DMSO or DMF.

[0021] The polymer or one of the blend components is a polymer modified with sulfinic acid groups (—SO₂H) and/or sulfinic acid salt groups (—SO₂Me) (Me=Li, Na, K, Rb, Cs or other mono- or di-valent metal cations). The choice of basic materials (polymeric sulfinic acids/sulfinic acid salts) is not limited; all polymeric or oligomeric sulfinic acids can be employed as the basic materials. The advantage of the cross-linking method according to the invention over the prior art is characterized by the following items:

[0022] The processes can be universally employed: all polymeric sulfinates and sulfinic acids can be cross-linked according to this process.

[0023] A wide variety of polymers can be blended with the polymeric sulfinic acids/sulfinic acid salts. Cross-linked polymer blends are obtained having blend morphologies (microstructures) and cross-linking densities which can be adjusted within wide ranges.

[0024] The cross-linked polymers have improved thermal and chemical resistance as compared with the basic substances.

[0025] The hydrolytic stabilities of the cross-linked polymers and membranes are substantially improved over the hydrolytic stabilities of other cross-linkings, for example, the cross-linking of polymeric sulfonates through sulfonamide links.

[0026] It has now been surprisingly found that the per se known disproportionation reaction can be made use of for cross-linking polymers by preparing sulfinic acid containing polymers according to a known method (Kerres, J.; Cui, W.; Neubrand, W.; Springer, S.: Reichle, S.; Striege, B.; Bigenberger, C.; Schnurnberger, W.; Severs, D.; Wagner, N.; Bolwin, K.: lecture (lector: J. Kerres), Euromembrane '95 Congress, Bath (UK), September 18 to 20, 1995, Proceedings, pages 1-284 (1995)) or other methods and cross-linking the same via the above disproportionation reaction, wherein other polymers may be added to the solution of the polymeric sulfinic acid in an appropriate solvent (for example, DMSO, DMAc, DMF, NMP) which polymers are then integrated in the forming polymeric network.

[0027] The microstructure of the generated cross-linked polymer blend depends on the compatibility of the blend components. If all blend components are compatible with each other, an interpenetrating network will form in which the polymeric chains of the blend components are entangled to such an extent that the blend components can hardly be separated any more. If the blend components are immiscible with each other, a microphase structure will form in which some of the components are dispersed in a continuum of the other components. The microphase structure is dependent on the kind of the blend components and the mixing ratio of the blend components. By appropriately selecting the blend components and their mixing ratio, cross-linked blend structures can be produced as desired.

[0028] The cross-linking process according to the invention can be employed, for example, for the preparation of cross-linked cation-exchange membranes. There are two ways of proceeding:

[0029] A polymeric sulfinic acid and a polymeric sulfonic acid are dissolved together in the same solvent (DMSO, DMAc, DMF, NMP) and then further processed.

[0030] A polymer the sulfinic acid groups of which have partially been oxidized to sulfonic acid groups (oxidation level 0 to 100%; the oxidation level can be adjusted through the quantity of oxidant added to the polymeric sulfinate) is dissolved in the solvent and then further processed.

[0031] This polymer solution is then cast on a substrate (glass plate, polished aluminum sheet, woven or non-woven fabric). Thereafter, the plate is placed in a drying oven and the solvent evaporated at elevated temperature, in particular above the boiling point of the solvent under the pressure conditions chosen, During the evaporation process, cross-linking of the polymeric sulfinic acid occurs. It has now been surprisingly found that the generated cross-linked cation-exchange membranes have a significantly increased thermal resistance as compared with the non-cross-linked polymeric sulfonic acid. Such cross-linked cation exchange membranes can now be employed to advantage in electromembrane processes, for example, in electrodialysis or in membrane fuel cells. In particular, their use is of advantage in such cases where it is necessary to operate at elevated temperatures and under severe chemical conditions since the cation-exchange membranes cross-linked according to the process invention exhibit reduced swelling in addition to an increased thermal stability as compared with the swelling of non-cross-linked cation-exchange membranes having the same cation-exchange capacity.

[0032] It has been shown that the yield of sulfone formation increases when the chain length of the alcohol is increased from CH₂—CH₂ to CH₂—CH₂—CH₂—CH₂ [P. Allen, J. Org. Chem. 7, 23-30 (1942)]. It has now been surprisingly found in preliminary experiments that this reaction can be made use of for cross-linking polymers which contain sulfinate groups.

[0033] Surprisingly, this cross-linking process could be successfully used for cross-linking ion-exchange membranes as well. The basic conception had been as follows: The Li salt of the polymeric sulfinate, for example, polysulfone Udel® (PSU)-Li sulfinate which is obtained by the reaction of metallated PSU with SO₂, is mixed with the Li salt of polymeric sulfonates, for example, PSU-Li sulfonate which is obtained by the oxidation of PSU-Li-sulfinate with oxidants, or with the Li salt of sulfonated PEK and the mixture is dissolved in di-polar-aprotic solvents, such as NMP, DMAc, DMSO or DMF. Then, an organic di- or oligohalogeno compound in which the halogen atoms can be substituted by nucleophilic substitution is added to the solution which is then stirred until complete mixing occurs. Thereafter, a thin film of the solution is prepared on a substrate, for example, on a glass plate, and the solvent is evaporated at: elevated temperature under reduced pressure or by using a forced convection. During the evaporation of the solvent, the organic di- or oligohalogeno compound reacts with the sulfinate residues of the polymers under cross-linking to disulfone links and with formation of halogenide anions. The polymeric chains of the polymeric sulfonate are incorporated in the network of the polymeric sulfinate. The network prevents the sulfonate component of the polymer blend membrane from swelling too much and significantly increases the chemical stability of the cross-linked membrane. As in the other cross-linking process mentioned above, another possibility is to partially oxidize a polymeric sulfinate, for example, PSU(SO₂Li), to the polymeric sulfonate (every oxidation level between 0 and 100% being possible) and then to react the partially oxidized polymer with the dihalogeno compound. The advantage of this process is that the whole polymer is involved in the cross-linking reaction, in contrast to the cross-linking of polymer blends where only the polymeric sulfinate component is cross-linked.

EXAMPLES

[0034] 1. Cross-linking of a Polymeric sulfinic acid by a disproportionation reaction between sulfine groups with integration of polymeric sulfonic acid in the generated network

[0035] 3 g of a polysulfone sulfinate (PSU Udel® (manufacturer: Amoco) with an average of one SO₂Li group per monomeric unit, 1.95=mol of SO₂Li/g of polymer) was dissolved in 30 g of DMF. Then, 4 g of DOWEX® cation-exchange resin, H form, 2.5 mmol SO₃H/g of polymer, was added for converting the polymeric sulfinate to the polymeric sulfinic acid since only the sulfinic acid underwent the disproportionation reaction. The polymeric sulfinic acid was prepared in situ because sulfinic acids are very unstable chemically. The polymeric sulfinate was fairly stable and could be carefully dried without decomposing. After the polymeric sulfinic acid had been obtained in this way, 6 g of polysulfone-sulfonic acid (PSU Udel® with an average of 0.9 SO₃H groups per monomeric unit, 1.7 mmol of SO₃H/g of polymer) was added to the solution. The polymeric sulfonic acid was dissolved with stirring. Then, the polymer solution was sheeted out on a glass plate or aluminum sheet to a thin film by means of a doctor blade. The plate with the film of polymer solution was placed in a drying oven, and the solvent was evaporated at temperatures between 80 and 120° C. under membrane pump vacuum. Thereafter, the plate with the membrane was immersed in deionized water. The membrane separated from the substrate.

[0036] Membrane data:

[0037] SO₃H content of the membrane: 1.33 mmol of SO₃H/g of polymer (theory: 1.35 mmol of SO₃H/g of polymer)

[0038] specific H⁺resistance of the membrane, measured in 0.5 N H₂SO₄ under a flow of direct current 50 Ωcm

[0039] swelling of the membrane in deionized water at room temperature: 33%.

[0040] 2. Cross-linking of a polymeric sulfinic acid salt by alkylation of sulfinic acid salts with integration of polymeric sulfonic acid in the generation network

[0041] A solution was prepared of 1-8 g of a PSU Udel®-Li sulfinate having an average of one sulfinate group per monomeric unit,

[0042] in 28 g of N-methylpyrrolidone (NMP). After dissolution of the sulfinate, 4-2 g of a PSU Udel®-Li sulfonate having an average of one Li sulfonate group per monomeric unit,

[0043] was added to the solution. After dissolution of the second component, 0.125 ml of 1,4-diiodobutane was added to the polymer solution. This was, stirred for about ¼ hour. After this reaction time, the solution already began to grow more viscous. An about 0.5 mm thick film was sheeted out on a glass plate from the polymer solution. Then, the glass plate was placed in a drying oven, and the solvent was evaporated at temperatures of 90-120° C. for 16 h under a reduced pressure of 850 mbar at first and finally 50 mbar. During this time, the membrane was cross-linked. After evaporation of the solvent, the polymer film was separated from the glass plate and treated first in a 1 N HCl solution at 70°C. for 24 h and then in distilled water at 70° for 24 h, with extraction of LiI and solvent residues from the membrane. Membrane properties: thickness: 0.121 mm ion-exchange capacity: 1.35 meq of SO₃H/g of polymer swelling (Na form, hot 36% water of 70° C., 24 h): surface resistivity 3.11 Ω · cm² (Na form, 0.5N Na₂SO₄): specific resistance 285 Ω · cm (Na form, 0.5N Na₂SO₄): perm. selectivity (Na form, 93.9% 0.5/0.1N NaCl):

[0044] In one embodiment of this invention, a method for preparing cross-linked polymers is provided using solutions of polymeric sulfinic acids or polymeric sulfinic acid salts having mono or polyvalent metal cations and removing the solvent from the solutions to initiate cross-linking of the polymers. In an alternative embodiment, the solution of polymers includes aryl or alkyl, di- or oligo-, halogeno compounds having one to twenty carbone. The halogen can be fluorine, chlorine, bromine or iodine.

[0045] In another embodiment of this invention, a method of preparing cross-linked polymers is provided using solutions of polymeric sulfinic acids or polymeric sulfinic acid salts having mono or polyvalent metal cations and cross-linking the polymers by a disproportionation reaction of the sulfinic acids.

[0046] In yet another embodiment of this invention, a method for preparing cross-linked polymers is provided using solutions of polymeric sulfinic acids or polymeric sulfinic acid salts having mono or polyvalent metal cations and using aryl or alkyl, di- or oligo-, halogeno compounds having one to twenty carbons and cross-linking the polymers by alkylating the sulfinic acid salts with the halogeno compounds. The halogen can be fluorine, chlorine, bromine or iodine.

[0047] All publications and patent applications mentioned in this specification are indicative of the level of skilled in the art to which this invention pertains. All publications and patent applications are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference. 

1. A covalently cross-linked polymer or a covalently cross-linked polymer blend or a covalently cross-linked polymer (blend) membrane, containing one or more polymers prepared from polymers containing at least sulfonate groups SO₂Me (Me=H, Li, Na, K, Cs or other metal cations or ammonium ions) and optionally added di-, tri- or oligofunctional haloalkanes or haloaromatics (halogen residue F, Cl, Br, I) as cross-linking agents and containing the following cross-links:


2. The covalently cross-linked polymer or covalently cross-linked polymer blend or covalently cross-linked polymer (blend) membrane according to claim 1 , containing one or more polymers, wherein the basic polymer or polymers bearing the functional groups are selected from the group consisting of polyethersulfones, polysulfones, poly-phenylsulfones, polyetherethersulfones, polyetherketones, polyether-etherketones, poly(phenylene ethers), poly(diphenylphenylene ethers), polypheenylenesulfides and/or copolymers containing at least lone of these components.
 3. The covalently cross-linked polymer or covalently cross-linked polymer blend or covalently cross-linked polymer (blend) membrane according to claim 2 , wherein the basic polymer or polymers bearing the functional groups are selected from the group consisting of poly-ethersulfones, polysualfones, polyphenylsulfones, polyetherethersulfones, poly(phenylene ethers), poly(diphenylphenylene ethers) and/or other polymers capable of being lithiated.
 4. The covalently cross-linked polymer or covalently cross-linked polymer blend or covalently cross-linked polymer (blend) membrane according to claim 1 , wherein the following functional groups are present, in addition to the cross-links generated from the sulfonate groups: SO₃Me, COOMe, PO₃Me₂, N(R₃)₃ ⁺(R₃=alkyl, aryl).
 5. The covalently cross-linked polymer or covalently cross-linked polymer blend or covalently cross-linked polymer (blend) membrane according to claim 4 , wherein SO₃Me is present as a functional group in addition to the cross-links generated from the sulfinate groups.
 6. The covalently cross-linked polymer or, covalently cross-linked polymer blend or covalently cross-linked polymer (blend) membrane according to claim 1 , wherein the following di- or oligohalogen compounds can be used as the cross-linking agents (X F, Cl, Br, I, x= (1-20):


7. The covalently cross-linked polymer or covalently cross-linked polymer blend or covalently cross-linked polymer (blend) membrane according to claim 6 , wherein the following di- or oligohalogen compounds are preferred as the cross-linking agents (X=F Cl, Br, I, x= 3-20)


8. The covalently cross-linked polymer or covalently cross-linked polymer blend or covalently cross-linked polymer (blend) membrane according to claim 7 , wherein the following dihalogen compounds are preferred as the cross-linking agents (X=Br, I, R₁=H, x=3-12x=3-12)


9. A process for the preparation of covalently cross-linked polymers, polymer blends or polymer (blend) membranes wherein at least one or more polymers bearing SO₂Me groups are dissolved, simnultaneously or successively, in a di- polar-aprotic solvent selected from N, N-dimethylformamide (DMF), N N-dimethylacetamide (DMAc), N-methylpyrrolidlinone (NMP), dimethyl sulfoxide (OMSO) or sulfolane, optionally followed by adding the cross-linking agent, followed by homogeneously dispersing the cross-linking agent in the polymer solution by stirring, optionally followed by filtering the polymer solution, optionally followed by degassing the polymer solution, followed by spreading the polymer solution as a thin film on a substrate (glass plate, metal plate, woven or non-woven fabric, etc.), followed by-removing the solvent by hearing at from 60 to 150° C. and/or by applying reduced pressure or in a recirculating dryer, optionally followed by separating the polymer film from the substrate, followed by aftertreating the polymer film in the following way, a) in 1 to 50% by weight aqueous alkali at T=RT to 95° C.; b) in completely desalted water at T=RT to 95° C; c) in 1 to 50% by weight aqueous mineral acid at T=RT to 95° C.; d) in completely desalted water at T=RT to 95° C.; in which one or more of the aftertreatment steps may optionally be omitted and/or the order of the aftertreatment steps may be changed.
 10. The process according to claim 9 , wherein the following polymers are dissolved in the polymer solution in addition to said polymer bearing the SO₂Me groups: one or more cation-exchange and/or anion-exchange polymers.
 11. The process according to claim 9 , wherein at least one polymer bearing both SO₂Me and SO₃Me groups is dissolved in said solvent.
 12. The process according to claims 9 and 10, wherein one or more different polymers containing SO₃Me, COOMe or PO₃Me₂ groups and/or one or more different polymers containing N(R₃)₃ ⁺Y⁻groups (Y =OH, F, Ci, Br, I) are dissolved in the polymer solution, in addition to said polymer bearing the SO₂Me groups.
 13. The process according to claim 11 , wherein one or more cation-exchange and/or anion-exchange polymers are dissolved in the solvent, in addition to said polymer bearing both SO₂Me and SO₃Me groups.
 14. Use of the membranes as defined in claim 1 for the electrochemical generation of energy.
 15. The use of the membranes according to claim 14 as a component of membrane fuel cells (H₂ or direct methanol fuel cells) at temperatures of from 0 to 180° C.
 16. The use of the membranes according to claims 14 and 15 in electrochemical cells.
 17. The use of the membranes according to claims 14 and 15 in secondary batteries.
 18. The use of the membranes according to claims 14 and 15 in electrolytic cells.
 19. The use of the membranes as defined in claim 1 in membrane separation processes, such as gas separation, pervaporation, perstraction, reverse osmosis, electrodialysis and diffusion dialysis. 